Barrier guards, shields, covers, screens and the like are among the oldest known safeguards for protecting personnel from the hazards of moving machinery. Their effectiveness derives from three properties: they prevent entry of the body into the zone of operation, they retain expelled missiles, and they define the safe from the unsafe portions of the machine. The need to access machinery leads to the removal or openings of barriers whereupon their concomitant protection is lost. Heretofore the shortcomings have been addressed by interlocks, which provide a connection between a barrier and the control or power system of the machinery to which the barrier is fitted. The interlock and the barrier with which it operates is designed, installed and adjusted so that until the barrier guard is closed into its protective state the interlock prevents the machinery from operating by interrupting the power medium, and also so that opening of the barrier causes the hazard to be eliminated before access is possible or it may be necessary that the barrier remain closed and locked until the risk of injury from the hazard has passed.
The barrier locking system wherein the barrier is to remain locked until the risk of injury from the hazard has passed is necessary when either 1) simply opening or removing the guard does not eliminate the hazard before access is possible or 2) opening a guard other than at predetermined points in the machine cycle may expose the hazard.
The guard locking system will normally consist of a timing device or motion or position sensing device and a guard locking device. These may be individual units or combined in one assembly. Variable conditions of operation of machinery produce variable amounts of run down and in these circumstances a timing device may be inappropriate to determine when the run down has reached a non-hazardous state since it has to be set for the longest run down time that might be expected. The variable time element may, however, be eliminated by the use of a motion or position sensing device, which allows the guard to be opened as soon as the hazard is no longer present.
Available on the market today are a number of position, motion, timing and guard locking devices that operate on various principles. Among motion and position sensing devices some may suffer from the disadvantage that they show poor response at low speed and are therefore acceptable only where residual motion after the guard has been opened could not cause injury. On the other hand, where injury could result from residual motion, more sensitive devices and or timing devices may be necessary. Examples of typical motion or position sensing devices are a) rotation sensing devices that may operate on centrifugal force, friction, eddy current generation, voltage generation, optical or electronic pulse generation b) photo-electric beam c) proximity devices or d) position switches or valves.
Timing device examples include a) mechanical, electrical or electronic clocks b) delay relays c) sequence valves d) threaded bolt or e) a dashpot.
Examples of typical guard locking devices are a) a captive-key unit b) a trapped-key unit c) mechanical bolt or d) shotbolts which may be solenoid operated, hydraulic or pneumatic.
The present invention relates to the testing of motion sensing devices that indicate zero speed or the cessation of motion. These devices actively monitor moving machine elements and are never benign when the machine is active. Such indicating devices may wear out or get out of adjustment or otherwise fail by prematurely signaling that motion has been arrested. This leads to unlatching of the barrier guards before the motion has ceased and before entry to the protected regions is safe. A statistically significant number of people will depend on the efficacy of the motion detectors to unlock guards when it may not be safe to do so.
To help prevent a false sense of security, it is desirable to improve the reliability of motion detectors and reliance thereon by regularly testing them. The dependence on zero speed systems is entirely analogous to the public's reliance on the “safety edges” on ordinary elevator doors.
Zero speed indicators may be completely removed from machines and tested by methods specified by the manufacturers. This procedure is practical only when infrequent inspections are anticipated and when the safety of the basic machine is not compromised by the removal of the motion indicator such as during a general machine shutdown.
The present invention describes a process whereby the motion detectors are frequently and automatically tested in situ while the machinery is in motion (and production) and while total personnel protection is assured. A further novel process is envisioned where the motion detectors are automatically tested in situ whenever the machine is shut down, such as when control systems stop switches are activated for lunch breaks, routine cleaning, maintenance or end-of-shift, when emergency stop devices are employed, when power disconnect is effected; or when latchless interlocked barriers are opened. In addition, when the motion detector indicates that the moving parts have stopped, it may be desired that absolute safety be insured by requiring a motion blocking member to be insertable and inserted between the now stopped moving parts before a guard protecting such parts can be opened.